Basics

PrimoPal’s Motor and Motion Control Products

A stepping motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft of a stepping motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

2. Advantages and Disadvantages of stepping motors

Advantages
• The rotation angle of the motor is proportional to the input pulses.
• The motor has full torque at standstill (if the windings are energized).
• Precise positioning and repeatability of movement since good stepping motors have an accuracy of 3 – 5% of a step and this error is non-cumulative from one step to the next.  Excellent response to starting/stopping/reversing.
• Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is mainly dependent on the life of the bearing.
• The motors response to digital input pulses provides open-loop control, making the system simpler and more cost efficient.
• It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
• A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages
• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.

3. Stepping motor types

There are three basic stepping motor types. They are:

• Variable-reluctance (VR)
This type of stepping motor has been around for a long time. It is probably the easiest to understand from a structural point of view. This type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.

• Permanent-magnet (PM)
The PM step motor is a low cost and low resolution type motor with typical step angles of 7.5° to 15° (48 – 24 steps/revolution). PM motors as the name implies have permanent magnets added to the motor structure. The rotor no longer has teeth as with the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.

• Hybrid (HB)
The hybrid stepping motor usually is more expensive than the PM stepping motor, but provides better performance with respect to step resolution, torque and speed. Typical step angles for the HB stepping motor range from 3.6° to 0.9° (100 – 400 steps per revolution). The hybrid stepping motor combines the best features of both the PM and VR type stepping motors. The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the air gap. This feature increases the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM types.

The two most commonly used types of stepping motors are the permanent magnet and the hybrid types. Generally speaking, the hybrid motor may be the better choice along with reducing cost, for it offers better performance with respect to step resolution, torque and speed. This catalog mainly introduces our Hybrid Stepper solutions for your motion control applications. A stepping motor can be a good choice whenever controlled movement is required. They can be used in applications where you need to control rotation angle, speed, position and synchronism. Because of the inherent advantages listed previously, stepping motors have found their place in many different applications.

4. Differences between bipolar and unipolar stepper motors

Bipolar motors are the strongest type of step motor, which have 4 or 8 lead wires. They have two coils inside, and there is no center tap in each winding. Stepping the motor round is achieved by energizing the coils and changing the direction of the current within these coils, which requires more complex electronics than a unipolar motor. Bipolar motors are typically used in applications required high torque at low speed.
Unipolar motors have two coils, and each one has a center tap. They are readily recognizable because they have 5, 6 or even 8 lead wires. 5 lead motors have both center taps connected. The main characteristic of unipolar motors is that you can step them without having to reverse the direction of current in any coil, which makes the electronics simpler. Because the center tap is used to energize only half of each coil at a time, unipolar motors generally have less torque than bipolar motors.
To be noted that, it is possible to drive 6 lead unipolar motors as bipolar motors if you ignore the center tap wires. In addition, the 8 lead motors split each coil in the middle, so you can wire the motor either as bipolar or unipolar.

5. Respective characteristics of four types of winding connection for eight lead wire step motors

• Unipolar connection and bipolar (half coil) connection don't give us great low speed torque, because of less turns. However, due to the low inductance, they hold the torque out to high speeds.
• Bipolar (series) connection uses the full coil, so it gives very good low speed torque. However, due to the high inductance, its torque drops off rapidly.
• Bipolar (parallel) connection also uses the full coil, so it gives good low speed performance. And its low inductance allows its torque to be held out to high speeds. But remember, current must be increased by 40% to get the advantages.

6. Difference and relationship between rated current and peak current

The rated current is what the motor is rated at, while the peak current refers to the amount of current the driver outputs.

• Full Stepping Drivers
When full stepping, the stepper driver simply outputs the Current value that the motor requires.
Peak Current = Rated Current

• Half Stepping Drivers
When half stepping, the rated Current and Peak Current are not equal. Unless the peak driver raises its output Current by 15%, the motor will not receive its rated Current, and is therefore not generating all of the torque that it’s capable of providing.
Peak Current = 1.15 x Rated Current

• Microstepping Drivers
When using a driver that is capable of doing microstepping, the definition of peak current becomes 1.414 times the rated current. Microstepping drivers are made differently in order to maximize their ability to drive the stepper motor. Therefore, stepper motors can handle up to their rated current multiplied by 1.414. This will not damage the motor because the power output is more or less the same.
Peak Current = 1.414 x Rated Current

7. Possible or advisable start/stop frequency

The maximum possible start/stop frequency depends on the frictional load or frictional torque but principally on the inert external masses and is specified as fs at no-load operation of the motor in the torque characteristics. If a straight line is placed between fs and the maximum torque, in this approach the possible start/stop speed can found very roughly at the intersection point of the torque and straight line where the acceleration torque Ma = J * a to the frictional torque must be added.
The recording of actual start/stop characteristics can only be incorporated through elaborate measurement results with different external inertial masses, which are then entered as a plurality of characteristics in the torque characteristics as parameters. As, on the other hand, the exact moments of inertia at the beginning of the project are often not yet available, we can determine the possible start frequencies experimentally in our laboratory with different inertial masses.

8. How fast can the hybrid stepper motor’s rotor rotate?

Most hybrid stepper motors are able to work around 3000 rpm or less. To be noted that, as motor speed increases, torque decreases. The stepper motor's low speed torque will vary directly with current. How quickly the torque falls off at faster speeds depends on the winding inductance and the drive circuitry it is attached to, especially the driving voltage.

9. Heat sources inside the stepper motor

Stepper motors are designed to run hot. The heat given off by the motor windings is due to simple resistive losses, eddy current losses, and hysteresis losses. If the heat is not conducted away from the motor adequately, the motor windings will overheat. The simplest failure which can be created is insulation breakdown, but it can also heat a permanent magnet rotor to above its curie temperature, the temperature at which permanent magnets lose their magnetization. This is a particular risk with many modern high strength magnetic alloys. Our Hybrid stepper motors are rated to run in an ambient temperature range of minus 20 degrees Celsius to 50 degrees Celsius and can have a temperature rise of 80 degrees Celsius.

10. Advice to deal with resonance

1) Microstep operation
The smaller the microstep, the smaller the resonance problems. Due to the smaller step angle, the overshoot angle is also reduced and the system has less pronounced resonance points. However, if no current compensation is provided or integrated in the microstep drive, a torque reduction of the motor occurs which can show up as a disadvantage in some applications.

2) Reducing the phase current
The higher the torque reserve, the higher the resonance excitation. Accordingly the resonance excitation is strongest during no-load operation and, therefore, brings the greatest resonance problems during testing. For this reason, tests should preferably be carried out in the application, because frictional torques is usually present here and hence the complete system is damped. In addition to the tendency to oscillate, however, the phase current reduction also minimizes the stiffness and must be taken into account in the positioning accuracy if no current compensation is integrated in the drive.

3) Changing the step frequency
The basic resonance of the stepper motors during no-load operation is full step at approx. 70-100 Hz and appears again more or less strongly at multiples or harmonics of the basic resonance. It is easiest to avoid the established resonance frequency if the process allows by choosing a frequency that is somewhat higher or lower (if necessary, through the interconnection of a gear or by changing the reduction gear ratio). Small deviations from the critical step frequencies already show good results.

4) Increasing the friction
The friction generally has a damping effect on the system and the overshoot angles become smaller. However, the reserve torque is reduced by this and the efficiency deteriorates.

5) Affixing a damper
The dampers reduce the overshoot angles and absorb the vibrant energy. The resonant frequencies are greatly reduced because the speed difference between the oscillating rotor and the external mass is reduced as well. Also the high running noise is greatly reduced by affixing a damper.

6) Changing the ramp gradient
At relatively low motor acceleration, it is possible to reach the points of resonance again during the run up time that make the system unstable once more. By contrast, a steep ramp has few sampling points the torque reserve is reduced by the high acceleration and the system has a more damping action.

7) Reducing the tendency to oscillate
In addition to the reduction of the tendency to oscillate with microstep, the danger of vibration declines with decreasing supply voltage due to the lower speed of the increase in current.